The BEOL memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory), and MRAM have a resistive device as a memory element. These memories are expected to have wide application because of their high access speed and non-volatility at power-off.
The memory device includes a multilayer magnetoresistive memory element in between a top electrode and a bottom electrode. The resistive memory element of MRAM is a Magnetic Tunnel Junction (MTJ) including a free layer and a fixed layer separated by a barrier layer. A magnetic moment of the free layer is manipulated by an electric current between the electrodes to be parallel or antiparallel to the fixed layer by an electric current between the electrodes. Whether the magnetic vector of the free layer is parallel or antiparallel to the fixed layer determines the low or high resistance state of the MTJ, which can be read using an electrical current that does not alter magnetic orientation. The two resistance states can be used as memory states “0” or “1”.
FIG. 1 illustrates a cross sectional view of the thin film layers at a selected stage of the fabrication process of MTJ memory cells according to the prior art. At the stage shown in FIG. 1, the unpatterned layers of MTJ film stack 20 have been deposited over the wafer onto the previously fabricated landing pads 21 or contact studs that connect to a control structures 19 like an FET and/or diode. The MTJ layer stack 20 includes unpatterned layers for the bottom electrode layer 22, lower magnetic layer 23, barrier layer 24, upper magnetic layer 25 and top electrode 26. Either one of the magnetic layers 23, 25 can be designed to work as a pinned layer with a fixed magnetic field and the other one acts as a free layer with a magnetization orientation that is manipulated by a vertical current that flow through the memory element between the electrodes. The barrier layer 24 is a dielectric film such as MgO or Al2O3 designed as efficiently manipulating TMR. The bottom electrode 22 and the top electrode 26 are primarily Ta or Ta alloys. A hard mask, which is not shown, can also be deposited as part of the standard fabrication process.
During the fabrication process, after the stage shown in FIG. 1, arrays of MTJ pillars 30 containing the patterned layers for the memory elements are formed on the wafer using conventional lithography and dry etching as shown in FIG. 2. The MTJ pillar 30 includes a bottom electrode 22′, a lower magnetic layer 23′, a barrier layer 24′, an upper magnetic layer 25′ and a top electrode 26′. The dry etching process often sputters metal and re-deposits it on MTJ pillar sidewall which is especially deleterious on the barrier layer. The metallic re-deposition material 27 can electrically short the pinned layer and the free layer across the sidewall area 28 of barrier layer 24′ rendering the device inoperable.
Sources of the re-deposition metal can be the MTJ stack itself or the landing pad metal that was deposited and patterned before the MTJ layer stack. After the etching process has removed the layers down through the barrier layer, the exposed sidewalls of the layers for the memory element become susceptible to being shorted with the re-deposited metal. The MTJ pillar is exposed to the process environment at this point in the process. After the unprotected material in bottom electrode layer is etched away, additional metal structures that were deposited and patterned below the MTJ layer are now exposed to the etching ambient. These previous structures include not only the MTJ landing pad but also peripheral circuitry. Thus there are several sources of metal that can be sputtered out and then re-deposited on the sidewall of the pillar.
The MTJ and bottom electrode layers are etched conventionally with ion milling or high-biased reactive ion etching where elements are mainly removed mechanically. Freed electrically conductive material generated by mechanical etching is easily re-deposited on the exposed sidewalls and can cause a short defect. However, during the etching process material is also being removed from the sidewalls, so it is the net result of these opposing processes that determines the final amount of re-deposited material that remains on the sidewalls. One variable is the slope of the sidewall, because the removal rate is faster on a shallow sloped sidewall. Also the re-deposition rate lowers with a decrease in remaining re-deposition source material.